The invention relates to high frequency induction heating in general and, more particularly, to induction heating apparatus having an improved capability for heat treatment of workpieces under controlled conditions of temperature, power density and/or frequency.
An induction heating apparatus conventionally includes a tank circuit fed with energy oscillating at the desired frequency and a coil applied to the workpiece for generating therethrough a high frequency electromagnetic field inducing active secondary currents into the workpiece under heat treatment.
Control of the induction heating apparatus is essential for an efficient operation and for adapting the existing equipment and power supply to a wide range of workpieces of different shape, geometry, and material.
A customary approach with induction heating apparatus has been to control the voltage, or the power applied to the coil circuit from the electrical power source. These methods have not been satisfactory because the final temperature for the workpiece treated is never obtained with sufficient precision for automatic control and manual adjustment has been required in general.
Where the final temperature is critical, the prior art has made use of closed loop feedback control by direct comparison of the actual temperature with the desired temperature as a reference. In such case, an error signal is generated which cases a change in the power supply.
Instead of controlling the power supply in regard to temperature, magnetic forces have also been used as the controlling parameter, but this requires a strict and precise control of the current passing through the induction coil for any quality standard by heat treatment to be achieved.
An object of the present invention is to provide coil current control in an induction heating apparatus.
The invention rests on the observation that neither the voltage, nor the power supplied to the tuned tank circuit is in direct relationship to the coil current.
Thus, for voltage control the coil current I.sub.C is given by the equation: ##EQU1## where L=coil inductance;
C=tuning capacitor; PA1 Vo=coil voltage; PA1 R=coil resistance; PA1 f=driving frequency; PA1 f.sub.o =resonant frequency of coil and tuning capacitors. PA1 I.sub.o =current fed to the tank circuit; PA1 .phi.=phase angle between current I.sub.o and voltage Vo.
For power control, the coil current I.sub.C is given by the equation: ##EQU2## where, in addition to the parameters of equation (1), Po=power applied to the tank circuit under Vo and I.sub.o ;
It appears that, in both instances, the coil current I.sub.C is dependent upon the driving frequency from the power supply as well as upon the impedance of the coil. Since all the aforementioned parameters are susceptible of varying during the heating process, a precise control cannot be achieved with either of these methods.
Accordingly, an induction heating apparatus has been conceived combining means for sensing the coil current directly and a closed loop for controlling the power supply in response to such sensing means.
Typically, the power supply is a static frequency converter, although it could be of the motor-generator type, an AC line power controller, a magnetic frequency multiplier, or a radio frequency generator, for instance.
Nevertheless, current control of heating induction apparatus gives rise to problems which are due to the nature of the heat treatment with this kind of apparatus. Whenever a workpiece is taken away from the tank an abrupt change of impedance takes place as seen by the active induction coils. This results in the control system calling for too much voltage. On the other hand, for a given setting of the control system the new workpiece might cause the system to abruptly call for too much power, which leads to an excessive current being drawn from the power supply.